1. Field of the Invention
This invention relates to a digital information modulating apparatus usable in various systems such as a system for recording a digital information signal or a digital video signal on a recording medium.
2. Description of the Prior Art
A known method of digital modulation for a digital VTR (video tape recorder) executes 24-25 conversion to enable the generation of prior signals for tracking. In the 24-25 conversion, a bit sequence representing information to be recorded is separated into groups each having 24 successive bits, and one bit is added to the head of every group. The conversion-resultant bit sequence is recorded on a magnetic tape after being subjected to NRZI encoding for recording.
The added bits are set to "1" or "0" in a manner such that the difference between the number of bits of "1" and the number of bits of "0" in each given length of the conversion-resultant bit sequence, that is, the digital sum variation (DSV), will periodically change to generate two pilot signals having different preset frequencies .omega.1 and .omega.2.
There are provided three different types F0, F1, and F2 of recording tracks on the magnetic tape for the pilot signals. An F0-type recording track is designed to generate no pilot signals. An F1-type recording track is designed to generate the pilot signal having the preset frequency .omega.1. An F2-type recording track is designed to generate the pilot signal having the preset frequency .omega.2.
On the magnetic tape, recording tracks of the different types are arranged as F0, F1, F0, F2, F0, . . . . Specifically, F1-type recording tracks and F2-type recording tracks are alternately present between F0-type recording tracks.
While a magnetic head scans an F0-type recording track, tracking control is executed so that the magnitudes of pilot signals leaked from neighboring tracks will be substantially equal. To enable easy detection of leaked pilot signals, an F0-type recording track is designed to correspond to a frequency spectrum having notches at the preset frequencies .omega.1 and .omega.2.